This invention relates to a method for heat treatment of titanium alloy or cast titanium alloy.
It is difficult to heat-treat titanium alloy because of both its high activity at an elevated temperature and the presence of a two-phase alloy. An abnormal structure develops if a temperature of solid solution treatment, which must be carried out at an elevated temperature, or a forging temperature is too high, or if the alloy is overheated by the heat generated at processing of the alloy. Once the abnormal structure has developed, it is difficult to effect its structural improvement even if it is subjected to successive heat treatment. The presence of the abnormal structure lowers the material characteristics of the alloy.
Heretofore, the titanium alloy having the abnormal structure has not been practically usable due to its lowered material characteristics unless it has been subjected to further processings such as forging, etc., resulting in a large increase in material cost, processing cost, etc. For example, when commercially available Ti-6A1-4V alloy is heated at a temperature over about 980.degree. C, an abnormal structure develops, as shown in FIG. 1a. The abnormal structure is an .alpha.' single phase, which can be converted to the normal, .alpha. or .alpha. + .beta. structure only when heat and processing are applied thereto, for example, by forging, etc. The .alpha.' single phase is liable to develop together with Widmanstatten structure due to a mishandling of the solid solution treatment or if heat buildup occurs during forging. These structures are not improved even by solid solution heat treatment and aging heat treatment, i.e. the ordinary treatments to increase the strength. That is, the ordinary heat treatments are ineffective in this situation, and thus good material characteristics of the titanium alloy cannot be completely obtained.
In the case of cast titanium alloy, it is cooled from a high temperature, and in the most cases the cooling rate is not constant. Due to this fluctuation in cooling rate, a mixed structure containing abnormal acicular .alpha. phases as a predominant structure is obtained. It is hard to carry out the solid solution heat treatment and heat aging treatment of the mixed cast structure to obtain an increased strength. The alloy in the mixed cast structure as such has a small elongation. Due to the small elongation, to date, the cast titanium alloy has been used, in the most cases, after only such a treatment as annealing has been applied thereto. That is, the cast titanium alloy has been used without fully improving its material characteristics. It is very difficult to carry out heat treatment of the structure of cast titanium alloy to obtain better material characteristics, and the structure of the cast Ti-6A14V alloy as such according to the ordinary method is shown in FIG. 2a. This structure contains abnormal acicular .alpha. phases as a predominant one, and unless heat and processing are applied thereto, for example, by forging, etc., a good effect cannot be obtained through application of the solid solution treatment and aging treatment, the ordinary heat treatment to obtain better material characteristics. Thus, to obtain material characteristics almost equivalent to those of forged or rolled materials, an improvement of structure is required for the cast Ti-6A1-4V alloy.